1. Field of Invention
This invention relates to a magnetic resonance imaging (MRI) system for obtaining a tomographic image of a slice of interest of an object under examination using the magnetic resonance phenomenon, and more particularly to a magnetic resonance imaging system capable of improving slicing characteristics.
2. Description of the Related Art
As is well known, magnetic resonance imaging is a method for obtaining chemical and physical microscopic distribution information of particles utilizing a phenomenon in which, when placed in a homogeneous static magnetic field (its intensity is H0), atomic nuclei having a specific magnetic moment, namely a group of nuclear spins, resonantly absorb high-frequency magnetic-field energy rotating in a plane orthogonal to the direction of the static magnetic field at an angular velocity .omega.0 defined by .omega.0=.gamma.H0 (.gamma. is the gyromagnetic ratio which is a constant specific to each type of atomic nucleus.)
As magnetic resonance imaging methods of imaging the spatial distribution of particular atomic nuclei (e.g. hydrogen atomic nuclei in water and fat), the projection reconstruction method by Lauterbur, the Fourier imaging methods by Kumar, Welti, Ernst or others, and the spin warp method (this is a modification of the Fourier method) by Hutchison et al. have been devised.
In such magnetic resonance imaging methods, a slicing technique is widely used which depends on the so-called selective excitation method to selectively excite magnetization in a slice of interest in a three-dimensional region so as to cause the magnetic resonance phenomenon, and to acquire magnetic resonance signals.
In the case of the selective excitation method the excitation of the magnetic resonance phenomenon is achieved in the following way:
An object placed in a uniform static magnetic field is further subjected to a linear magnetic field gradient acting as a slicing gradient magnetic field whose intensity varies linearly in the direction orthogonal to the slice-plane of interest. Owing to the magnetic field gradient of the magnetic resonance frequency, which corresponds to the intensity of the magnetic field, linearly varies in the direction orthogonal to the slice plane. Under this condition, in order to excite and refocus the magnetization in the slice, the slice is subjected to a pulsed high-frequency magnetic field, or a high-frequency pulse which has a frequency bandwidth corresponding to the slice thickness and a center frequency corresponding to the magnetic resonance frequency at the center of the slice thickness. In this case, an exciting high-frequency pulse adapted for rotating the magnetization (vector) by 90 degrees by resonant absorption is referred to as a 90-degree selective excitation pulse (90.degree. SEP), while a high-frequency pulse adapted for rotating the magnetization by 180 degrees (reversing) or refocusing it is referred to as a 180-degree selective excitation pulse (180.degree. SEP).
When the magnetization in the slice is excited and refocused by selective excitation pulses, the satisfactory selectivity of the slice (called slicing characteristic) cannot be always obtained. That is, the normally used selective excitation pulses are not such optimum ones as to rotate by 90 degrees or 180 degrees the magnetization in the slice of interest alone. To obtain an optimum slicing characteristic, or a sharp slicing characteristic, an attempt is made to optimize the waveform of the selective excitation pulses which includes the shape of an envelope of amplitude modulation and a high-frequency waveform of phase modulation. Or, another attempt is made to use a composite selective excitation pulse system employing a set of a plurality of excitation pulses. As described, to obtain a sharp slicing characteristic various means have been employed conventionally, but satisfactory results have not been obtained as yet.
It is important that the rotation phases of the magnetizations (magnetization vectors) are made coincident with each other at points located in a slice by the selective excitation of magnetic resonance and the echo refocusing. However, in the case of a general object, or an object in which the distribution of density is not uniform with respect to the direction orthogonal to the slice plane, the above-described magnetization phase condition cannot be obtained. The variations in phase of the magnetization in the slice plane generate image artifacts or false images, thus degrading the image quality.
Accordingly, in the conventional magnetic resonance imaging system, not only the poor slicing characteristic but also the artifacts due to the variations in phase of the magnetization in the slice plane degrade the image quality. In recent years the signal-to-noise ratio and the spatial resolution of magnetic resonance images have been substantially improved. Thus, solving the problems resulting from the slicing characteristic and the variations in phase of the magnetization in the slice will be the key to an improvement in the magnetic resonance image quality.
On the other hand, attempts have also been made to measure blood flow utilizing magnetic resonance phase information in the magnetic resonance imaging system. The variations in phase of the magnetization in the slice is an important factor which reduces the accuracy of measurement.
With the existing MRI system, as described above, there are problems of degradation in the image quality and poor accuracy of the measurements due to poor slicing characteristic and variations in the phase of the magnetization in a slice.